This invention relates to methods and apparatus for scanning a charged particle beam, such as an ion beam, and, more particularly, to scanners which operate over a wide range of charged particle beam energies. The invention is particularly useful in ion implanters, but is not limited to such use.
Ion implantation has become a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of a wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded in the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam is distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement.
The ion implanter may include an electrostatic or magnetic scanner for deflecting the ion beam over the surface of the wafer being implanted. The scanner may deflect the ion beam in one dimension or in two dimensions, depending on the design of the system. Both electrostatic and magnetic scanners are well known to those skilled in the art.
An electrostatic scanner includes one or more sets of scan plates. The scan plates of each set are spaced apart to define a gap, and the ion beam is directed through the gap. A scan voltage, which may have a sawtooth waveform, is applied to the scan plates. The scan voltage produces between the scan plates an electric field which deflects the ion beam in accordance with the scan voltage waveform. Electrostatic scanners are disclosed in U.S. Pat. No. 4,922,106 issued May 1, 1990 to Berrian et al and U.S. Pat. No. 4,751,393 issued Jun. 14, 1988 to Corey, Jr. et al.
Magnetic scanners typically include magnetic polepieces and a coil, which constitute an electromagnet. The magnetic polepieces are spaced apart to define a gap, and the ion beam is directed through the gap. A scan current applied to the coil produces in the gap a magnetic field which deflects the ion beam. By varying the current supplied to the coil in accordance with a desired scan waveform, the beam is scanned magnetically. A magnetic scanner is disclosed in U.S. Pat. No. 4,367,411 issued Jan. 4, 1983 to Hanley et al.
Prior art beam scanners have had fixed scan plates or fixed electromagnets for operation over a prescribed range of beam energies. In electrostatic scanners, the scan plate spacing is selected to produce the desired deflection at the maximum beam energy. At low beam energies, the beam expands due to space charge effects, and only part of the beam passes between the scan plates. As a result, the beam current delivered to the wafer is reduced, and implant times are increased, often to an unacceptable extent. In some cases, the beam current is reduced to a negligible level, and the implant cannot be performed. Prior art electrostatic scanners which utilize fixed scan plates typically operate over a range of approximately one order of magnitude in energy, for example, 40 keV to 400 keV.
The implanted depths of the dopant material is determined, at least in part, by the energy of the ions implanted into the semiconductor wafer. In accordance with the trend in the semiconductor industry toward smaller, higher speed devices, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. State of the art semiconductor devices require junction depths less than 1000 angstroms and may eventually require junction depths on the order of 200 angstroms or less. Very low implant energies, on the order of 1-10 keV, are required to achieve such shallow junctions. At the opposite end of the energy range, high energies, on the order of 1 MeV or greater, are required for device features such as electrical isolation from the silicon substrate. Thus, a wide range of implant energies is required.
It is desirable to provide ion implanters which can operate over a wide range of ion energies, so that one implanter may be utilized for all or most implants in a semiconductor process. However, prior art beam scanners have not been capable of operation over a wide range of energies for the reasons discussed above. At low energies, beam transmission may be unacceptably low, and at high energies, beam deflection may be insufficient. Accordingly, there is a need for improved beam scanners that operate over a wide range of beam energies with high beam transmission and with the required beam deflection.
According to a first aspect of the invention, apparatus is provided for scanning a charged particle beam. The apparatus comprises scan elements spaced apart by a gap for passing a charged particle beam, a scan signal generator coupled to the scan elements for generating scan signals for scanning the charged particle beam in a scan pattern having a scan origin, and a position controller for positioning the scan elements based on at least one parameter of the charged particle beam. For example, the scan elements may be positioned based on the energy of the charged particle beam.
In one embodiment, the scan elements comprise electrostatic scan plates for electrostatic deflection of the charged particle beam, and the scan signal generator comprises a scan voltage generator. In another embodiment, the scan elements comprise magnetic polepieces and a magnetic coil for energizing the magnetic polepieces, and the scan signal generator comprises a scan current generator for energizing the magnetic coil.
The position controller may comprise means for positioning the scan elements to achieve a desired position of the scan origin for given parameter values of the charged particle beam. The scan elements may be positioned to achieve a fixed position of the scan origin for different parameter values, such as different energies, of the charged particle beam. Where the scan elements are electrostatic scan plates, a fixed position of the scan origin may be achieved by moving the scan plates upstream with respect to the charged particle beam as the spacing between the scan plates is increased. In particular, the scan plates may be translated along linear paths disposed at equal and opposite angles with respect to an axis of the charged particle beam. In another approach, the scan plates may be rotated as the spacing between the scan plates is changed. The scan plates may have a continuous range of positions or may have two or more discrete positions.
According to another aspect of the invention, apparatus is provided for scanning a charged particle beam. The apparatus comprises first scan elements spaced apart by a first gap for passing a charged particle beam, second scan elements spaced apart by a second gap for passing the charged particle beam, a scan signal generator coupled to the first scan elements and the second scan elements for generating scan signals for scanning the charged particle beam in a scan pattern having a scan origin, and a scan signal controller for controlling the scan signals supplied from the scan signal generator to the first scan elements and the second scan elements based on at least one parameter of the charged particle beam. For example, the scan signals may be controlled based on the energy of the charged particle beam.
In one embodiment, the first scan elements and the second scan elements each comprise scan plates for electrostatic deflection of the charged particle beam, and the scan signal generator comprises a scan voltage generator. In another embodiment, the first scan elements and the second scan elements each comprise magnetic polepieces and a magnetic coil for energizing the magnetic polepieces, and the scan signal generator comprises a scan current generator for energizing the magnetic coil.
The scan signal controller may comprise means for controlling the scan signals supplied to the first scan elements and the second scan elements to achieve a desired position of the scan origin for given parameter values of the charged particle beam. In one configuration, the scan signals supplied to the first and second scan elements are controlled to achieved a fixed position of the scan origin for different parameter values, such as different energies, of the charged particle beam. In another configuration, the scan signals supplied to the first and second scan elements are controlled to change the effective length of the first and second scan elements. The scan signal controller may adjust the ratio of the scan signals supplied to the first scan elements and the second scan elements.
According to a further aspect of the invention, a method is provided for scanning a charged particle beam. The method comprises the steps of directing a charged particle beam between spaced-apart scan elements, energizing the scan elements for scanning the charged particle beam in a scan pattern having a scan origin, and controlling positions of the scan elements based on at least one parameter of the charged particle beam.
According to yet another aspect of the invention, a method is provided for scanning a charged particle beam. The method comprises the steps of directing a charged particle beam between spaced-apart first scan elements and spaced-apart second scan elements, applying scan signals to the first scan elements and the second scan elements for scanning the charged particle beam in a scan pattern having a scan origin, and controlling the scan signals supplied to the first scan elements and the second scan elements based on at least one parameter of the charged particle beam.
According to yet another aspect of the invention, apparatus is provided for scanning an ion beam. The apparatus comprises two or more pairs of scan plates for scanning the ion beam and a scan generator for applying scan voltages to the two or more pairs of scan plates for scanning a high energy beam and for applying scan voltages to a subset of the two or more sets of scan plates for scanning a low energy beam. Unused scan plates are electrically grounded. An effective length over which electrical fields are applied to the ion beam is reduced for scanning a low energy beam.